1. Technical Field
The present invention relates to nitride-based compound semiconductor and a nitride-based compound semiconductor device.
2. Related Art
Nitride-based compound semiconductors, such as GaN-based semiconductor, have greater bandgap energy and breakdown voltage than silicon-based material, and therefore nitride-based compound semiconductor can be used to manufacture a semiconductor device with high breakdown voltage that can operate in high-temperature environments. Accordingly, the nitride-based compound semiconductors are expected to be used for power devices, such as inverters and converters, replacing the silicon-based material.
In the power devices, high OFF breakdown voltage is an important parameter for determining the maximum output of a transistor. In order to achieve the high OFF breakdown voltage, it is necessary to decrease the leak current and improve the buffer breakdown voltage.
The GaN-based semiconductor is generally hetero-epitaxially grown on a heterogeneous substrate, and therefore the GaN-based semiconductor includes lots of lattice defects including dislocations and point defects such as nitrogen vacancies. In particular, if a silicon substrate is used, the lattice constant difference between GaN and silicon is approximately as large as 17% and the thermal expansion coefficient difference is approximately as large as 56%. Therefore, dislocations occur with a high density exceeding 1010 cm−2. A GaN-based semiconductor device with such high-density dislocation exhibits large leak current and low breakdown voltage.
One of the techniques for increasing the breakdown voltage is to increase the resistance of a buffer layer formed directly on the substrate. As a method to increase the resistance of the buffer layer, an auto-doping method has been proposed, in which carbon contained in an organic metal, which is the raw material, is used as a dopant when using the metal organic chemical vapor deposition (MOCVD) (see, for example, Japanese Patent Application Laid-open No. 2007-251144, J. E. Northrup, Appl. Phys. Lett., vol. 78, p. 2200 (2001), and J. W. P. Hsu, M. j. Manfra, R. J. Molnar, B. Heying, and J. S. Spec, Appl. Phys. Lett., vol. 81, p. 79 (2002)).
When considering reliability of a device, it is preferable that the leak current be prevented from increasing not only at the initial time of using the device but also for a long term period of operation exceeding 1000 hours. However, in a device whose buffer layer has increased resistance due to carbon doping, such as the device disclosed in Japanese Patent Application Laid-open No. 2007-251144, even though the leak current at the initial time of using the device, i.e. the initial value of the leak current, is no greater than a predetermined value, the leak current increases with time.
The present invention has been achieved in view of the above problems, and it is an object of the present invention to provide nitride-based compound semiconductor and a nitride-based compound semiconductor device that have high long-term reliability.